1. Field of the Invention
The present invention relates to a voltage regulator including an overcurrent protection circuit.
2. Description of the Related Art
FIG. 3 is a circuit diagram of a drooping type overcurrent protection circuit in a conventional voltage regulator. The overcurrent protection circuit generates such a short circuit current as shown in FIG. 4 when short circuit occurs at its output terminal, and is therefore called a drooping type overcurrent protection circuit.
The circuit is composed of: an error amplifier 11 for amplifying a difference voltage between a reference voltage of a reference voltage circuit 10 and a voltage Va at a node or dividing point A of bleeder resistors 13 and 14 for dividing a voltage Vout of the voltage regulator; and an output transistor 12. When an output voltage of the error amplifier 11 is Verr, an output voltage of the reference voltage circuit 10 is Vref, and Vref≦Va, then Verr decreases. When Vref≦Va, Verr increases.
When Verr decreases, at the output transistor 12 which is a P-ch MOS transistor in this case, a gate-source voltage increases and ON resistance decreases. Thus, an output voltage Vout increases. When Verr increases, the ON resistance of the output transistor 12 increases and the output voltage decreases, thereby keeping the output voltage Vout constant. When a load resistance 20 decreases and an output current increases, the gate-source voltage of the output transistor 12 increases. A current flowing into a transistor 15 having a gate voltage common with the output transistor 12 also increases. With the larger current flowing into the transistor 15, a current flowing into a resistor 16 increases. When the current is equal to or larger than the predetermined value, the voltage increase by the resistor 16 exceeds a threshold value of a transistor 17. The transistor 17 is turned ON and a current flows into a resistor 18. Then a transistor 19 is turned ON, the gate voltage of the output transistor 12 increases, and the output voltage decreases, whereby overcurrent protection operates.
With respect to the drooping type overcurrent protection circuit in the conventional voltage regulator, irrespective of the output voltage, the current generated upon overcurrent protecting operation is constant. The output voltage is controlled to be low, but the output current is kept high and a load imposed on the circuit is kept large (see JP 2003-029856 A (page 6, FIG. 3)).